1. Field of the Invention
The present invention relates to a vehicle speed control system, and particularly to a vehicle speed control system that controls at least one of an engine, a transmission and a brake, in accordance with control parameters computed based on running environment ahead of the vehicle.
2. Description of the Related Art
Japanese unexamined patent publication No. H04-236699 discloses a system for realizing stable running by: extracting a road profile (for example, curved track) from map information stored in a navigation system that detects or estimates a driving route; setting a target velocity in accordance with properties of the curved track; and decelerating a vehicle before entering the curved track.
In a conventional navigation system mounted on a vehicle, a driving route is detected or estimated based on information of its own position of the vehicle received from the GPS (Global Positioning System, or satellite navigation system) and on a map DB (Data Base) stored in DVD-ROM, hard disk and the like. In order to realize the system disclosed in the above-mentioned patent document, high accuracy is required in the GPS and the map DB. However, in the system using the GPS, accuracy becomes markedly poor depending on the geographic features, due to occurrence of multipath or failure in satellite capture. In order to solve this problem, Japanese unexamined patent publication No. 2004-351994 discloses a method in which judgment is made on reliability of a road profile obtained from other information than the map information (for example, information obtained from objects installed along a road, such as white lines (lanes), delineators, guardrails and the like, detected using a camera, a radar and the like), and when the reliability is judged to be high, a target velocity calculated from the map information is corrected based on the road profile.
However, in the method disclosed in this patent document, the target velocity is corrected based on a result of reliability judgment on a road profile, and therefore, running stability may become poor depending on a road profile on which the vehicle is running. For example, suppose a vehicle enters a slow curve that leads to a sharp curve. When an imaging device, such as a camera, is used for recognizing lanes and the road profile is obtained based on a result of this recognition, and if the image is clear and the result of lane recognition is excellent, reliability of the road profile obtained by the camera is judged to be high. However, if the road is flanked by walls, the sharp curve is hidden behind the walls and the camera cannot capture the sharp curve ahead. Therefore, the judgment based merely on the result of lane recognition of the slow curve results in the judgment that no deceleration is required. In addition, there arises a problem in processing of the image obtained by the camera, that accuracy in computing information of distant road point becomes poor due to a limitation in resolution. For instance, if the target velocity is corrected in accordance with the road profile obtained by the camera, deceleration control is not performed, even though a target velocity computed from the map information stored in the navigation system indicates requirement of deceleration. Thereafter, when the vehicle enters the section where the sharp curve ahead can be detected based on the result of lane recognition by the camera, a target velocity required for deceleration is computed. However, a distance from the vehicle to the sharp curve is very short, and rapid deceleration is urged. The deceleration may not be completed before the vehicle reaches the sharp curve entrance, and running stability becomes markedly low. Therefore, it has been demanded to attain velocity control with high running stability, even in such a situation.
Therefore, it would be desirable to provide a method for highly stable velocity control which utilizes both map information stored in the navigation system and other information than the map information (for example, information of lane recognition by an imaging device, such as a camera).